The present invention relates to a battery supply control unit that controls a current supply from a battery to a load, and more particularly to a battery supply control unit capable of preventing electric discharge by a dark current from the battery.
According to considerations of the inventor of the present invention, a vehicle such as an automobile includes a plurality of loads such as an engine electronic control unit, meter electronic control unit, automatic transmission electronic control unit, memory provided unit and the like. To drive such plural loads during driving of the vehicle, an electric power needs to be supplied and therefore, the vehicle is provided with a battery.
The battery supply control unit provided to the vehicle controls a supply of electric current from the battery to the plural loads.
FIG. 3 shows a structure of a circuit which is an example of a conventional battery supply control unit which has been analyzed by the present inventor.
Referring to FIG. 3, the battery supply control unit 114a controls a supply of electric power of a battery 111 applied via a fuse 113 to a plurality of loads 121a to 121n. The battery supply control unit 114a comprises a vehicle mounted relay 115, a transistor 117 and a controller 119. The vehicle mounted relay 115 has an electromagnetic coil 116a and a contact piece 116b. When the contact piece 116b is closed (ON), loads 121a to 121n are supplied with electric power from battery 111.
With the above structure, when a vehicle is driving, ignition (IG) is turned ON or its engine is turned ON, so that ignition signal IGS (H level) is input to a controller 119 through a terminal B.
Next, the controller 119 turns ON the transistor 117 so that a current flows from the battery 111 to the transistor 117 through the electromagnetic coil 116a. Thus, causing the contact piece 116b to close, i.e., is turned ON. As a result, electric power from the battery 111 is supplied to the plurality of loads 121a to 121n so that a predetermined current of about several amperes (A) flows through these loads.
On the other hand, during non-driving conditions, ignition is turned OFF or the engine is turned OFF, so that the ignition signal IGS (L level) is input to the controller 119. Then, the controller 119 outputs L level ignition signal IGS for a predetermined period of time interval. The predetermined period of time interval may, for example, be several days to about one month.
Because the transistor 117 is kept ON for a predetermined period of time interval, a dark current I of several tens mA flows from the battery 111 to the transistor 117 through the electromagnetic coil 116a. At the same time, because the contact piece 116b is ON for the predetermined period of time interval, electric power of the battery 111 is supplied to the plurality of the loads 121a to 121n. 
After the predetermined period of time interval elapses, the controller 119 turns OFF the transistor 117 so that no dark current flows from the battery 111 to the electromagnetic coil 116a. Thus, the contact piece 116b is opened (OFF) thereby interrupting the supply of the electric power from the battery 111 to the plurality of the loads 121a to 121n. 
However, such a structure can not prevent a large discharge of the battery. Further, because the dark current may continue to flow, the discharge period of the battery 111 is also quickened.
FIG. 4 shows a circuit structure diagram of another example of a conventional battery supply control unit.
In FIG. 4, a battery supply control unit 114b comprises a keep relay 123, a controller 125, a reset transistor 126 and a set transistor 127. The keep relay 123 has a 2-winding coil 124a and a contact piece 124b. The keep relay 123 allows an electric power to be supplied to the 2-winding coil 124a only when the contact piece 124b is turned from ON to OFF or from OFF to ON, and after this changeover, the OFF state or ON state is maintained. The reset transistor 126 is connected to an end of one winding coil of the 2-winding coil 124a and the set transistor 127 is connected to an end of the other winding coil.
With such a structure, if H level ignition signal is input to the controller 125 through the terminal B, the controller 125 turns ON the set transistor 127 so that a current flows from one end of one winding coil to the other end thereby the contact piece 124b being turned ON.
On the other hand, during non-driving, if the L level ignition signal is input to the controller 125, the controller 125 turns ON the reset transistor 126 so that a current flows from the other end of the other winding coil to one end thereby the contact piece 124b being turned OFF.
Therefore, with such a structure, because a current flows to the 2-winding coil 124a only when contact piece 124b is turned from ON to OFF or from OFF to ON through the keep relay 123, power consumption is reduced.
However, because keep relay 123 generally has a low holding force for closing the contact point, the contact piece 124b may accidentally be turned OFF because of vibration or the like during vehicle driving. Thus, there is a need for improvement in the connection reliability of conventional battery supply control units during vehicle driving. Further, in general, keep relays are often not suitable for large currents. Therefore, to make it match such a large current, a more expensive keep relay is often required.